Detection circuit for mixed asynchronous and synchronous memory operation

ABSTRACT

A memory access mode detection circuit and method for detecting and initiating memory access modes for a memory device The memory access mode detection circuit receives the memory address signals, the control signals, and the clock signal and generates a first mode detection signal in response to receipt of the memory address signals or a first combination of control signals. An first mode initiation signal is generated a time delay subsequent to the detection signal to initiate the first mode memory access operation. In response to receipt of a second combination of control signals and an active clock signal, the memory access mode detection circuit further generates a second mode detection signal to initiate a second mode memory access operation and to suppress generation of the first mode detection signal, thereby canceling the first mode memory access operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. patent application Ser.No. 11/354,786, filed Feb. 14 2006, which is a continuation of pendingU.S. patent application Ser. No. 11/129,150, filed May 13, 2005, whichis a continuation of U.S. patent application Ser. No. 10/357,862, filedFeb. 3, 2003, issued Jul. 19, 2005 as U.S. Pat. No. 6,920,524 B2.

TECHNICAL FIELD

The present invention is related generally to the field of integratedcircuits, and more particularly, to circuitry for detecting asynchronousand synchronous memory operations in a memory device.

BACKGROUND OF THE INVENTION

A class of memory devices called pseudo-static memory are typicallymemory devices that are functionally equivalent to static random accessmemory (SRAM) devices, but have a memory core based on conventionaldynamic random access memory (DRAM) cells. In general, these memorydevices can be operated in the same manner one would operate aconventional SRAM. As is well knows in the art, a major distinctionbetween the two types of memory cells is that DRAM memory cells need tobe periodically refreshed to maintain the stored data, whereas SRAMmemory cells do not. Consequently, pseudo-static memory devices includeinternal refresh circuitry to perform the necessary refresh operationsof the DRAM memory core. However, refresh operations are transparent tothe user, so that the devices appear as not needing refresh operations.

Although there appear to be disadvantages in employing a DRAM memorycore over an SRAM memory core because of the need for periodic refreshoperations to be performed, there are, however, significant advantagesin other respects. For example, memory density for a DRAM memory arraycan be much greater than that for a SRAM memory array. In the case of aDRAM memory cell, only one transfer gate and a storage device, typicallya capacitor, is necessary to store one bit of data. In contrast,conventional SRAM memory cells can have as many as six transistors permemory cell. Additionally, the simple structure and smaller size of DRAMmemory cells translate into less complicated manufacturing processes,and consequently, lower fabrication costs when compared to the SRAMmemory cell. In turn, memory devices employing DRAM memory cores areconsiderably cheaper than SRAM memory devices having equivalent memorycapacities.

In an effort to integrate a DRAM memory core into a memory device thatis functionally equivalent to an SRAM device, the operationaldifferences between the two types of memory need to be addressed. Forexample, one difference, as previously discussed, is that DRAM memorycells need to be refreshed periodically or the data stored by the memorycells will be lost. As a result, additional circuitry must be includedin the memory device to support refresh operations, but should maintainrefresh transparency to the user.

Another difference between an SRAM memory core and a DRAM memory core isthat once a memory access operation for a conventional DRAM memory corehas begun, the entire access cycle needs to be completed or data will belost. That is, a DRAM access cycle begins with a row of memory cells inthe array being activated, and the respective charge state of the memorycells for the activated row are sensed and amplified. A particularmemory cell is selected by coupling a column to an input/output line.Consequently, the memory cell at the intersection of the activated rowand the selected column is accessed. At this time, data can be read fromor written to the particular memory cell. Following the read or writeoperation, the row of memory cells is deactivated, thus, the chargestates that were initially sensed and amplified are stored by therespective capacitors of the memory cells. As is generally known, theprocess of sensing the charge state of the memory cells is destructive,Unless the DRAM access cycle is completed by amplifying the charge stateand properly deactivating the row, the data stored by the memory cellsof the activated row will be lost.

In contrast, for a conventional asynchronous SRAM memory device, theSRAM sense operation is non-destructive and does not have the same typeof access cycle as a conventional DRAM memory device. Consequently,random memory addresses may be asserted to the SRAM memory devicewithout timing restriction, and data is always expected to be returnedin a certain time thereafter. This time is typically referred to as theaddress access time t_(AA).

Yet another difference between memory devices having an SRAM memory coreand those having a DRAM memory is that access times for DRAM memorycores are generally longer than the access times for SRAM memory cores.Asynchronous access of a DRAM memory core requires more time to providevalid data because of the time required to complete the access cycle,Although conventional DRAM devices often provide advanced access modesto decrease average access times, such as page mode access, valid memoryaddresses must nevertheless be provided for each data access. As aresult, the minimum access time of a memory device will be limited bythe setup time for providing valid and stable memory addresses, which insome cases, can take a relatively long time.

Synchronous DRAM (SDRAM) devices, which operate according to a periodicclock signal and have pipelined architectures to provide shorter averageaccess times than asynchronous DRAM devices. Memory access times forSDRAM devices are generally lower because the pipelining of internalmemory operations allow for different stages of a DRAM memory accessoperation to be executed in parallel, as well known in the art. Thisallows for new memory commands to be initiated prior to the completionof previous memory commands. As a result, conventional SDRAM devices canprovide modes of operation that cannot be replicated by theirasynchronous DRAM counterparts. For example, SDRAM devices have a databurst mode where new data can be output each period of a clock signalafter an initial memory access without the need to provide any memoryaddresses other than for the first memory location. That is, data storedat the starting memory location is accessed, and data from sequentialmemory locations are thereafter accessed without the need to providefurther memory addresses.

Despite the aforementioned disadvantages, in many instances, it is stilldesirable to employ memory devices having a DRAM memory core for theadvantages previously discussed. Therefore, it is desirable to havecircuitry that can be employed in a memory device that provides theasynchronous functionality of an SRAM device, and which accommodates thescheduled events of accessing a DRAM memory core. Moreover, in manyapplications, it is desirable for the circuitry to automatically detectwhether an asynchronous or synchronous memory access operation isrequested without the use of a flag or dedicated control signal thatinstructs the memory device to expect an asynchronous or synchronousmemory access operation. In this manner, a memory device having suchcircuitry can be used as a companion device with existing types ofconventional memory devices.

SUMMARY OF THE INVENTION

One aspect of the invention provides an interface circuit for amixed-mode memory device. The interface circuit includes an asynchronousoperation command detection circuit operable to generate a delayedmemory operation activation pulse in response to receipt of commandsignals corresponding to a request for an asynchronous memory operation.The delayed memory operation activation pulse is delayed by a minimumdelay time relative to receipt of the command signals. The interfacecircuit further includes a synchronous operation command detectioncircuit operable to generate a memory operation activation pulse inresponse to receipt of command signals corresponding to a request for asynchronous memory operation. The synchronous operation commanddetection circuit is further operable to suppress generation of adelayed memory operation activation pulse by the asynchronous operationcommand detection circuit. The memory operation activation pulsegenerated by the synchronous operation command detection circuit isgenerated sooner after receipt of the command signals than the minimumtime delay.

Another aspect of the invention provides a mode detection circuit forinitiating a memory access operation in a memory device receiving memoryaddress signals and control signals. The mode detection circuit includesfirst and second mode detection circuits. The first mode detectioncircuit is operable to generate a first mode detection signal to beprovided at a first output node in response to receipt of memory addresssignals and receipt of control signals requesting an asynchronous memoryaccess operation. The second mode detection circuit is operable togenerate a second mode detection signal to be provided at a secondoutput node in response to receipt of an active clock signal and receiptof control signals requesting a synchronous memory access operation. Adelay circuit is coupled to the first and second mode detectioncircuits. The delay circuit is operable to provide a delayed modedetection signal in response to a last received first mode detectionsignal and is further operable to interrupt provision of the delayedmode detection signal in response to the second mode detection signal.An output circuit is coupled to the second mode detection circuit andthe delay circuit and is operable to provide an activation signal toinitiate a memory access operation in response to the delayed modedetection signal or the second mode detection signal.

Another aspect of the invention provides a method for initiating amemory operation in response to receiving command signals. The methodincludes generating a memory operation activation pulse in response tothe command signals requesting an asynchronous memory operation. Thememory operation activation pulse is generated at a time delay followingreceipt of the most recently received command signals requesting anasynchronous memory operation. The method further includes generatingthe memory operation activation pulse in response to the command signalsrequesting a synchronous memory operation. The memory operationactivation pulse is generated sooner after receipt of the commandsignals than the time delay associated with an asynchronous memoryoperation. A memory operation is initiated in response to the memoryoperation activation pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an asynchronous/synchronousdetection circuit according to an embodiment of the present invention.

FIG. 2 is a functional block diagram of an embodiment of a delay circuitthat can be used in the detection circuit of FIG. 1.

FIG. 3 is a signal timing diagram illustrating various signals appliedto the detection circuit of FIG. 1.

FIG. 4 is a functional block diagram of a portion of a memory deviceincluding an asynchronous/synchronous detection circuit according toanembodiment of the present invention.

FIG. 5 is a functional block diagram of a computer system includingmemory devices of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings, which form apart hereof, and in which are shown, by way of illustration, specificexemplary embodiments in which the invention may be practiced. In otherinstances, well-known circuits, control signals, and timing protocolshave not been shown in detail in order to avoid unnecessarily obscuringthe invention. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention. Otherembodiments may be utilized and modifications may be made withoutdeparting from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims.

FIG. 1 illustrates an asynchronous/synchronous mode detection circuit100 according to an embodiment of the present invention. The detectioncircuit 100 can be employed in a memory device that is functionallyequivalent to an SRAM device, but uses a DRAM memory core. As will beexplained in more detail below, a significant benefit provided byembodiments of the present invention is the automatic detection ofsynchronous/asynchronous operation. The detection circuit 100 alsoallows for the memory device to be operated synchronously as well.Included in the detection circuit 100 is asynchronous mode detectioncircuitry 110 to which address signals ADDR<0:n> and control signals areprovided. As illustrated in FIG. 1, the control signals provided to theasynchronous mode detection circuitry 110 include conventional controlsignals, such as a chip enable signal CE*, an address valid signal ADV*,an output enable signal OE*, and a write enable signal WE*. The asterisk“*” indicates that the respective control signal is an active lowsignal, that is, the signal is considered active when at a LOW logiclevel. The ADDR<0:n> signals and the CE*, ADV*, OE*, and WE* signals areconventional, and are known by those of ordinary skill in the art.

Further included in the detection circuit 100 is synchronous modedetection circuitry 120, which receives the CE*, ADV*, OE*, and WE*signals. The synchronous mode detection circuitry 120 also receives aperiodic clock signal CLK that is used by the synchronous mode detectioncircuitry 120 to synchronize operation of the memory device. Forexample, the synchronous mode detection circuitry 120 includes controlsignal latches (not shown) that latch the logic state of the CE*, ADV*,OE*, and WE* signals in response to transitions of the CLK signal, suchas the rising edge of the CLK signal, the falling edge of the CLKsignal, or in some embodiments, on both the rising and falling edges ofthe CLK signal. The asynchronous mode detection circuitry 110 and thesynchronous mode detection circuitry 120 are of conventional designknown by those of ordinary skill in the art.

It will be appreciated the previously described control signals havebeen provided by way of example, and that alternative control signalsmay be provided to the asynchronous mode detection circuitry 110 and thesynchronous mode detection circuitry 120 without departing from thescope of the present invention.

A refresh timer 130 is also included in the detection circuit 100. Therefresh circuit 130 is coupled to receive a pulse PULSE_ASYNC from theasynchronous mode detection circuitry 110 and a pulse PULSE_SYNC fromthe synchronous control circuitry 110. As will be explained in moredetail below, the refresh timer 130 generates an output pulse PULSE_OUTa time delay t_(d) after the filling edge of the last (i.e., mostrecent) PULSE_ASYNC pulse from the asynchronous mode detection circuitry110. However, in the event a PULSE_SYNC pulse is generated by thesynchronous mode detection circuitry 120 prior to the time delay t_(d)elapsing, the refresh timer 130 will be reset and deactivated to preventa PULSE_OUT pulse from being generated by the refresh timer 130. Atwo-input Boolean logic OR gate 140 is coupled to receive the PULSE_OUTand PULSE_SYNC pulses from the refresh timer 130 and the synchronousmode detection circuitry 120, respectively. An output of the OR gate 140is coupled to provide an activation pulse ACT_PULSE to conventional DRAMactivation circuitry 150 in order to initiate an access operation in theDRAM memory core (not shown).

By way of background, a memory access operation is initiated in aconventional SRAM device by enabling the SRAM device with an active (LOWlogic level) CE* signal, and asserting a memory address. In someapplications, an ADV* signal is used to indicate to the SRAM that thememory address is valid, and can be latched to initiate the memoryoperation. The type of access, that is, whether a read operation or awrite operation is executed, is controlled by the logic levels of theother control signals. For example, a read operation is typicallyexecuted in response to the WE* signal having a HIGH logic state at thetime the memory address is asserted. In contrast, a write operation isexecuted in response to the WE* signal having a LOW logic state at thetime the address is asserted. With respect to a read operation for anSRAM device, read data is expected to be returned from the memory devicea certain time after the asserted memory address has been held valid forthe minimum time. The maximum time required for the read data to bereturned is typically referred to as the address access time t_(AA). Inthe event a new address is asserted before the access operation iscomplete, the previous access operation is aborted, and a new accessoperation is initiated for the memory location of the newly assertedaddress.

As previously discussed, in a conventional DRAM memory core, accessingmemory locations in the DRAM memory core is a destructive operation.That is, when a row of memory is accessed, the data stored by the memorycells of that row are essentially erased, and must be written back tothe memory cells prior to the completion of the memory access cycle. Asa result, it is typically the case that conventional DRAM memory coresare not well suited for use in memory devices that will be accessed inthe manner of an SRAM device because of the asynchronous manner in whichmemory access operations can be initiated in the SRAM device. That is,although the previously described situation of asserting a new memoryaddress prior to the completion of a memory access operation is easilyaccommodated by conventional SRAM memory cores, this is not the casewith a conventional DRAM memory core. As previously explained, thedestructive nature of an access operation for a conventional DRAM memorycore requires that a memory access operation that is initiated must beallowed to complete or risk loss of data. The detection circuit 100 canbe employed to accommodate the use of a DRAM memory core with aconventional SRAM memory interface.

The detection circuit 100, however, can be used in a memory devicehaving a conventional DRAM memory core to convert randomly scheduledaddress transitions, which conventionally used to initiate SRAM accessoperations, into scheduled events that are suitable for conventionalDRAM memory cores. The detection circuit 100 further provides amechanism for memory devices having conventional DRAM memory cores to beaccessed both asynchronously in the manner of an SRAM address interfaceas well as synchronously to provide the benefits of conventionalsynchronous DRAM devices. The operation of the detection circuit 100will be discussed with respect to an asynchronous access operation of aconventional SRAM address interface, followed by a synchronous memoryaccess operation, and then a memory access operation where anasynchronous access operation is immediately followed by a synchronousaccess operation. A memory access operation that includes transitioningfrom an asynchronous to a synchronous memory access operation can bereferred to as a mixed mode operation. Embodiments of the presentinvention automatically detect transitions in mixed mode operations.That is, detection of asynchronous and synchronous memory accessoperations can be made without any externally supplied flags thatinstruct a memory device to expect either an asynchronous or synchronousmemory access operation.

As previously discussed, a memory access to an SRAM device is initiatedupon activating the memory device by a LOW CE* signal and asserting amemory address. Thus, upon receiving a newly asserted memory address anda LOW CE* signal, the asynchronous mode detection circuitry 110generates a PULSE-SYNC pulse that is provided to the refresh timer 130to initiate the time delay t_(d). After the time t_(d) has elapsed, therefresh timer 130 generates a PULSE_OUT pulse that is provided throughthe OR gate 140 as the ACT_PULSE pulse to the DRAM activation circuits150. In response to receiving the ACT_PULSE, the DRAM activationcircuits 150 initiate an access operation to the memory location in theDRAM memory core corresponding to the memory address asserted to theasynchronous mode detection circuitry 110.

The value of the refresh timer 130 will now be explained. Theasynchronous mode detection circuitry 110 generates a PULSE_ASYNC pulsein response to receiving a new memory address, regardless of whether thenew memory address is being asserted prior to the completion of a memoryaccess cycle. The refresh timer 130 inserts a time delay t_(d) ofsuitable length to ensure that any previously initiated memory accessoperation will have sufficient time to complete. In the event therefresh timer 130 is reset by the PULSE_ASYNC pulse generated by theasynchronous mode detection circuitry 110 before t_(d) elapses, the timedelay t_(d) is reset so that the delay is measured from receipt of themost recent PULSE_ASYNC pulse. By selecting the time delay t_(d) to belong enough to allow a memory access operation to complete, the refreshtimer 130 ensures that a memory access operation will not be interruptedprior to its completion. That is, since the time t_(d) is always resetupon the receipt of a PULSE_ASYNC pulse, the refresh timer 130 ensuresthat an ACT_PULSE (i.e., a PULSE_OUT pulse) will not be provided to theDRAM activation circuits 150 in response to the assertion of a memoryaddress any sooner than the time t_(d) has elapsed, which, as previouslydiscussed, is selected to allow a memory access operation to complete.In a particular embodiment of the present invention, the delay t_(d) isapproximately 25 ns, which still allows for a memory device employing aDRAM memory core to have an access time t_(AA) of 60 ns.

FIG. 2 illustrates a delay circuit timer 220 that can be included in therefresh timer 130 (FIG. 1). The delay circuit 220 includes a pluralityof delay stages 240. Each delay stage 240 has a delay input and a resetinput, and further has a delay output. As will be explained in moredetail below, a reset circuit (not shown) also included in the refreshtimer will be used to reset the delay circuit timer 220 in response toreceiving a PULSE_SYNC pulse from the synchronous mode detection circuit120. However, the reset circuit, which can be designed by thoseordinarily skilled in the art, will not be discussed with respect to thedelay circuit timer 220 in order to avoid unnecessarily complicating thedescription of the delay circuit timer 220.

In operation, a delay stage 240 provides an output signal that issimilar to the signal applied to the delay input except that it isdelayed by a time t_(dd). A first delay stage 240 receives thePULSE_ASYNC signal at both its delay input and reset input. Subsequentdelay stages 240 are coupled such that the delay input is coupled to thedelay output of the previous delay stage 240. The reset input of each ofthe delay stages 240 is coupled to receive the PULSE_ASYNC signal, andthe delay output of the last delay stage 240 is coupled to a first inputof a two-input NOR gate 250. A second input of the NOR gate 250 iscoupled to receive the PULSE_ASYNC signal. An output of the NOR gate 250is coupled to a conventional pulse generator 254 through an inverter252. The pulse generator 254 generates the pulse PULSE_OUT in responseto the falling edge of the signal output by the inverter 252. ThePULSE_OUT signal, as previously mentioned, is provided to the DRAMactivation circuits 150 through the OR gate 140 to start an accessoperation to a conventional DRAM memory core.

In operation, the delay circuit 220 generates a PULSE_OUT pulse a timedelay t_(d) after the falling edge of the most recent PULSE_ASYNC pulse.The time delay t_(d) is approximately the sum of the delay t_(dd) ofeach delay stage 240. In an effort to simplify explanation of the delaycircuit 220, any gate delays have been ignored. However, it will beappreciated that some time will be added to the time delay t_(d) becauseof the gate delays. When the delay circuit 220 receives an PULSE_ASYNCpulse, on the falling edge of the PULSE_ASYNC pulse, the delay circuitbegins counting the time delay t_(d). That is, for the first delay stage240 in the chain, its delay output will go LOW t_(dd) after the fallingedge of the PULSE_ASYNC pulse. The delay output of the second delaystage 240 will go LOW t_(dd) after the falling edge of the delay outputof the first delay stage 240. Thus, the falling edge of the PULSE_ASYNCpulse will trickle through the chain of delay stages 240 until beingapplied to the input of the NOR gate 250. Note that during this time,the output of the inverter 252 has remained HIGH. Not until the delayoutput of the last delay stage 240 goes LOW, which occurs t_(d) afterthe falling edge of the PULSE_ASYNC signal, will the output of theinverter 252 go LOW. When this does occur, the pulse generator 254 thengenerates a PULSE_OUT pulse.

In the event a second PULSE_ASYNC pulse is received by the delay circuit220 before the t_(d) timing count has elapsed, the delay stages 240 ofthe timing chain are reset by causing the delay output of each of thedelay stages 240 to go HIGH again in response to the new PULSE_ASYNCpulse. As a result, the t_(d) countdown will begin again in response tothe falling edge of the new PULSE_ASYNC pulse, as previously described.In effect, the pulse generator 254 will not generate a PULSE_OUT pulseuntil t_(d) after the falling edge of the last PULSE_ASYNC pulseprovided to the delay circuit 220.

A more detailed description of the delay circuit 220 is provided incommonly assigned, co-pending U.S. patent application Ser. No.10/102,221, entitled ASYNCHRONOUS INTERFACE CIRCUIT AND METHOD FOR APSEUDO-STATIC MEMORY DEVICE to Lovett et al., filed Mar. 19, 2002. Itwill be appreciated, however, that the refresh timer 130 can includedelay circuitry other than that shown in FIG. 2, that is well known bythose of ordinary skill in the art.

With respect to a synchronous memory access operation, the detectioncircuit 100 includes synchronous mode detection circuitry 120 that canbe used to initiate synchronous memory access operations of aconventional DRAM memory core. With reference to FIG. 1, the combinationof the logic levels of control signals and provision of a periodic clocksignal CLK to the synchronous mode detection circuitry 120 initiate suchan operation. The synchronous mode detection circuitry 120 isconventional in design, and the design of suitable synchronous modedetection circuitry 120 is known by those of ordinary skill in the art.Upon receiving the correct combination of logic signals of the controlsignals, and provision of a CLK signal, the synchronous controlcircuitry generates a PULSE_SYNC pulse that is provided to the refreshtimer 130 and the OR gate 140. The resulting PULSE_SYNC pulse isprovided to the DRAM activation circuits 150 through the OR gate 140 asthe ACT_PULSE, which initiates memory access to the DRAM memory core. Itwill be appreciated that the synchronous mode detection circuitry 120provides internal control signals (not shown) in addition to thePULSE_SYNC pulse shown in FIG. 1 in order to execute a synchronousmemory access operation. However, the internal control signals areconventional in nature, and have not been shown in order to avoidunnecessarily obscuring the invention.

As an example of a combination of control signals that can be used toinitiate a synchronous memory access operation, in a particularembodiment of the present invention, a synchronous memory writeoperation is requested when the CE* and WE* signals are at a logic LOW,the OE* signal is at a HIGH logic level, and an active CLK signal isprovided to the synchronous control circuitry. The requested memoryaddress is asserted, and the ADV* signal is LOW to indicate that thememory address is valid and should be latched by an address buffer (notshown). After initiation of the synchronous memory write operation, theADV* and WE* signals can return to a HIGH logic level. A burst writeoperation can continue as long as the CE* signal is at a LOW logic leveland an active CLK signal is provided to the synchronous mode detectioncircuit 120.

As previously mentioned, the PULSE_SYNC pulse generated by thesynchronous mode detection circuitry 120 is provided to the refreshtimer 130 as well as to the OR gate 140. As will be explained below, thePULSE-SYNC pulse is provided to reset the refresh timer 130 before aPULSE_OUT pulse can ever be generated by the refresh timer 130. Instead,the PULSE_SYNC pulse provided to the OR gate 140 by the synchronous modedetection circuitry 120 is used as the ACT_PULSE pulse to initiate asynchronous memory access operation immediately.

Operation of the detection circuit 100 during a mixed mode operationwill be explained with reference to the timing diagram of FIG. 3. Thetiming diagram illustrates the relative timing of various signalsapplied to the detection circuit 100 in transitioning from anasynchronous memory read operation to a synchronous memory writeoperation. The timing diagram of FIG. 3 is being provided by way ofexample, and should not be interpreted as limiting the scope of thepresent invention to a particular embodiment.

The asynchronous memory access cycle is initiated at a time T0 byproviding a LOW logic level CE* signal (i.e., chip enable), asserting amemory address and strobing the ADV* signal LOW to indicate that thememory address input is valid. The asynchronous mode detection circuitry110 (FIG. 1) generates a PULSE_ASYNC pulse in response to the assertionof the memory address, which begins a time delay t_(d) 330 of therefresh timer 130. As shown in the timing diagram of FIG. 3, the timedelay t_(d) 330 is approximately 25 ns. When the time delay t_(d) 330elapses, a PULSE_OUT pulse is generated at a time T1 by the refreshtimer 130 and provided through the OR gate 140 to the DRAM activationcircuits 150 as an ACT_PULSE pulse to initiate a memory access operationin the DRAM memory core. After the time t_(AA) elapses, that is, theminimum access time for the memory device, the OE* signal (i.e., outputenable) is made active by changing it to a logic LOW level at a time T2.In response, valid read data 340 is provided at the input/output (IO)terminals of the memory device. At a time T3, the IO terminals areplaced in a high impedance state by returning the OE* signal to a HIGHlogic level, and the memory device is put in a standby state by changingthe CE* signal to a HIGH logic level. The time T3 represents the end ofthe asynchronous memory access cycle.

In the present example shown in FIG. 3, the transition from theasynchronous memory access mode to a synchronous memory access modeoccurs on the rising edge of the CLK signal following a time T4, thatis, when the CE* signal becomes active, or more specifically, when theCE* signal goes LOW. As will be discussed in more detail below, when theCE* signal becomes active at the time T4, it is assumed that anasynchronous memory access operation will be executed until a risingedge of the CLK signal in combination with the ADV* signal is detected.At that time, the asynchronous memory access operation is cancelled, anda synchronous memory access operation is initiated instead. It will beappreciated by those ordinarily skilled in the art that where the timedelay t_(d) 330 is approximately 25 ns, a maximum time of 25 ns canelapse from the time the CE* signal becomes active at the time T4 andthe time when the rising edge of the CLK signal is detected. Otherwise,the asynchronous memory access operation that is assumed to have beeninitiated will begin in the DRAM memory core before the synchronousmemory access operation.

At the time T4, the memory device is enabled by changing the logic levelof the CE* signal to LOW, and a write operation is indicated by strobingthe WE* signal LOW. A memory address is also asserted and the ADV*signal is strobed LOW to signal that the address input is valid. At atime T5, a synchronous write operation in the DRAM memory core isinitiated when, in response to a rising edge of the CLK signal, thesynchronous mode detection circuitry 120 (FIG. 1) detects the active CE*and WE* signals and generates a PULSE_SYNC pulse that is provided to theDRAM activation circuits 150 through the OR gate 140. The memory addressis latched on the rising edge of the CLK signal as well. At a time T6,the ADV* and WE* are returned to a HIGH logic level, while the CE*signal remains at a LOW logic level to indicate that the requestedsynchronous memory write operation should not be terminated.

At the time T4, as part of enabling the memory device, the asynchronousmode detection circuitry 110, which also received the CE*, ADV*, andaddress signals, will generate a PULSE_ASYNC pulse. The PULSE_ASYNCpulse is generated in response to the CE* signal becoming active at thetime T4, and an asynchronous memory access operation is started on therefresh timer 130 (FIG. 1). Before the asynchronous memory accessoperation is initiated in the DRAM memory core by the generation of thePULSE_OUT pulse after the time delay t_(d) elapses, a PULSE_SYNC pulsegenerated by the synchronous detection circuitry 120 at a time T5cancels the queued asynchronous memory access operation. The PULSE_ASYNCpulse is automatically generated in response to the assertion of thememory address. As a result, the refresh timer will begin the timedelay. Consequently, in order to prevent a PULSE_OUT pulse from beinggenerated and interrupting the synchronous memory write operation, whichas previously discussed is initiated at the time T5, the refresh timer130 is reset and disabled by the PULSE_SYNC pulse generated by thesynchronous mode detection circuitry 120. As a result, a PULSE_OUT pulseis never generated by the refresh timer 130.

At a time T7, write data 360 present on the IO terminals is latched andwritten to the location in the DRAM memory core corresponding to thememory address latched at the time T5. As previously discussed, whilethe CE* signal remains at a LOW logic level, the synchronous memorywrite operation will continue. The synchronous memory access operationscan be terminated by returning the CE* signal to a HIGH logic level, andtransition back to an asynchronous memory access can accomplished bydisabling the CLK signal.

FIG. 4 illustrates a portion of a memory device 500 according to anembodiment of the present invention. The memory device 500 is anasynchronous pseudo-static SRAM that includes a conventional DRAM memoryarray 502. The memory device 500 can be operated asynchronously orsynchronously. The memory device 500 includes a command decoder 506 thatreceives memory commands through a command bus 508 which generatesinternal control signals within the memory device 500 to carry outvarious memory operations. The command bus 508 is also coupled to anasynchronous/synchronous detection circuit 512 that is in accordancewith an embodiment of the present invention. Examples of the signalsreceived over the command bus 508 include CE*, ADV*, OE*, and WE*signals, as previously described. However, it will be appreciated bythose ordinarily skilled in the art that changes to the particularsignals provided to the memory device 500 over the command bus 508 willnot depart from the scope of the present invention. Row and columnaddress signals are provided to an address buffer 510 of the memorydevice 500 through an address bus 520, as well as to the detectioncircuit 512.

As previously described, the detection circuit 512 generates anACT_PULSE pulse to initiate an access operation to the memory array 502.Although previously described as being provided to DRAM activationcircuits 150 (FIG. 1), as illustrated in FIG. 4, the ACT_PULSE pulse isprovided to the command decoder 506 to initiate a memory accessoperation in FIG. 5. It will be appreciated, however, that the ACT_PULSEsignal can be provided to alternative or additional functional blocks ofa conventional memory device without departing from the scope of thepresent invention.

The row and column addresses are provided by the address buffer 510 fordecoding by a row address decoder 524 and a column address decoder 528,respectively. Memory array read/write circuitry 530 are coupled to thearray 502 to provide read data to a data output buffer 534 via ainput-output data bus 540. Write data are applied to the memory array502 through a data input buffer 544 and the memory array read/writecircuitry 530. The command controller 506 responds to memory commandsapplied to the command bus 508 to perform various operations on thememory array 502. In particular, the command controller 506 is used togenerate internal control signals to read data from and write data tothe memory array 502. The data read from the memory array 502 aretransferred to the output buffer 534 and provided on data input/output(IO) lines 550. In a write operation, the addressed memory cell isaccessed and data provided on the IO lines 550 to the data input buffer544 are stored in the memory array 502.

FIG. 5 is a block diagram of a computer system 600 including computercircuitry 602 that contains the memory device 500 of FIG. 4. Thecomputer circuitry 602 performs various computing functions, such asexecuting specific software to perform specific calculations or tasks.In addition, the computer system 600 includes one or more input devices604, such as a keyboard, coupled to the computer circuitry 602 to allowan operator to interface with the computer system. Typically, thecomputer system 600 also includes one or more output devices 606 coupledto the computer circuitry 602, such output devices typically being adisplay device. One or more data storage devices 608 are also typicallycoupled to the computer circuitry 602 to store data or retrieve data.Examples of storage devices 608 include hard disks and non-volatilememory. The computer system 600 also includes a wireless communicationlink 610 through which the computer circuitry can send and receive datathrough a wireless medium. The computer circuitry 602 is typicallycoupled to the memory device 500 through appropriate address, data, andcontrol busses to provide for writing data to and reading data from thememory.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. For example, the embodiment ofthe present invention described in FIG. 1 includes a two-input OR gate140 that provides the ACT_PULSE pulse to the DRAM activation circuits150 to initiate a memory access operation based on either a PULSE_OUTpulse from the refresh timer 130 or a PULSE_SYNC pulse from thesynchronous mode detection circuitry 120. However, in an alternativeembodiment of the present invention, the OR gate 140 will not beincluded, and the PULSE_OUT and PULSE_SYNC pulses will be provided toDRAM activation circuitry directly to initiate either an asynchronousmemory access operation or a synchronous memory access operation,respectively. Moreover, the embodiment of FIG. 1 illustrates separatefunctional blocks for the asynchronous mode detection circuitry 110,synchronous mode detection circuitry 120, refresh timer 130, OR gate 140and DRAM activation circuits 150. However, it will appreciated by thoseordinarily skilled in the art that the various functional blocks may becombined into different arrangements than that shown in FIG. 1, andstill remain in the scope of the present invention. Accordingly, theinvention is not limited except as by the appended claims.

1. An interface circuit for a mixed-mode memory device, comprising: anasynchronous operation command detection circuit operable to receive acommand signal and generate an asynchronous memory operation activationsignal in response to receipt of the command signal corresponding to arequest for a memory operation; a synchronous operation commanddetection circuit operable to receive a command signal and generate asynchronous memory operation activation signal in response to receivingthe command signal; a delay circuit coupled to the asynchronousoperation command detection circuit and the synchronous operationcommand detection circuit, the delay circuit operable to receive theasynchronous memory operation activation signal and to generate anoutput signal after a time delay from receiving the asynchronous memoryoperation activation signal, the delay circuit further operable toprevent the output signal from being generated in response to receivingthe synchronous memory operation activation signal before the time delayends; and a logic circuit coupled to the synchronous operation commanddetection circuit and the delay circuit, the logic circuit operable togenerate a memory access signal in response to receiving the outputsignal and the memory access signal in response to receiving thesynchronous memory operation activation signal.
 2. The interface circuitof claim 1 wherein the command signal corresponding to a request for asynchronous memory operation include a clock signal.
 3. The interfacecircuit of claim 2 wherein the synchronous operation command detectioncircuit is configured to generate the synchronous memory operationactivation signal in response to receipt of the command signalcorresponding to a request for a synchronous memory operation and arising edge of the clock signal.
 4. The interface circuit of claim 1wherein the asynchronous operation command detection circuit comprises:a signal generator operable to generate the output signal in response toreceipt of the command signals corresponding to a request for anasynchronous memory operation; and the delay circuit.
 5. The interfacecircuit of claim 4 wherein the delay circuit is further operable tosuppress generation of the output signal in response to the generationof the synchronous memory operation activation signal by the synchronousoperation command detection circuit.
 6. The interface circuit of claim 4wherein the delay circuit comprises: a chain of delay stages, each delaystage having first and second inputs and an output and operable to delaya falling edge of a signal applied to either of the first or secondinputs from propagating to its output by a stage time delay; and a resetcircuit coupled to the chain of delay stages and operable to disable thechain of delay stages to suppress output of the memory operationactivation signal in response to generation of the memory operationactivation signal by the synchronous operation command detectioncircuit.
 7. A memory device, comprising: an array of memory cells; amemory array access circuit coupled to the array of memory cells foraccessing the memory array; and a memory access mode detection circuitcoupled to receive command signals, the memory access mode detectioncircuit comprising: an asynchronous operation command detection circuitoperable to generate an asynchronous memory operation activation signalin response to receipt of command signals corresponding to a request foran asynchronous memory operation; a synchronous operation commanddetection circuit operable to generate a synchronous memory operationactivation signal in response to receipt of command signalscorresponding to a request for a synchronous memory operation; a refreshcircuit coupled to the asynchronous operation command detection circuitand the synchronous operation command detection circuit, the refreshcircuit operable to receive the asynchronous memory operation activationsignal and to generate an output signal after a time delay fromreceiving the asynchronous memory operation activation signal, therefresh circuit further operable to prevent the output signal from beinggenerated in response to receiving the synchronous memory operationactivation signal before the time delay ends; and a logic circuitcoupled to the synchronous operation command detection circuit and therefresh circuit, the logic circuit operable to generate a memory accesssignal in response to receiving the output signal and the memory accesssignal in response to receiving the synchronous memory operationactivation signal.
 8. The memory device of claim 7 wherein the commandsignals corresponding to a request for a synchronous memory operationinclude a clock signal.
 9. The memory device of claim 8 wherein thesynchronous operation command detection circuit of the memory accessmode detection circuit is configured to generate the synchronous memoryoperation activation signal for the memory array access circuit inresponse to receipt of command signals corresponding to a request for asynchronous memory operation and a rising edge of the clock signal. 10.The memory device of claim 7 wherein the asynchronous operation commanddetection circuit of the memory access mode detection circuit comprises:a signal generator operable to generate a signal in response to receiptof the command signals corresponding to a request for an asynchronousmemory operation; and the refresh circuit, the refresh circuit includinga delay circuit coupled to the signal generator, the delay circuit beingoperable to generate the output signal a time delay following receipt ofa last received signal from the signal generator.
 11. The memory deviceof claim 10 wherein the delay circuit is further operable to suppressgeneration of the output signal in response to generation of the outputsynchronous memory operation activation signal by the synchronousoperation command detection circuit.
 12. The memory device of claim 10wherein the delay circuit comprises: a chain of delay stages, each delaystage having first and second inputs and an output and operable to delaya falling edge of a signal applied to either of the first or secondinputs from propagating to its output by a stage time delay; and a resetcircuit coupled to the chain of delay stages and operable to disable thechain of delay stages to suppress output of the output signal inresponse to generation of the synchronous memory operation activationsignal by the synchronous operation command detection circuit.
 13. Amethod for initiating a memory operation in response to receivingcommand signals, comprising: in response to the command signals,generating an asynchronous memory operation activation pulse; inresponse to the command signals, generating a synchronous memoryoperation activation pulse; in response to receiving the asynchronousmemory operation activation pulse generating an output signal after atime delay from receiving the asynchronous memory operation activationpulse; using a refresh circuit, inhibiting the generation of the outputsignal in response to receiving the synchronous memory activation pulsebefore the time delay ends; and generating a memory access signal inresponse to receiving the synchronous memory operation activation pulseand the output signal.
 14. The method of claim 13 wherein the commandsignals requesting a synchronous memory operation include a clocksignal.
 15. The method of claim 14 wherein generating the synchronousmemory operation activation pulse following receipt of the commandsignals requesting a synchronous memory operation comprises generatingsynchronous the memory operation activation signal in response toreceipt of command signals requesting a synchronous memory operation anda rising edge of the clock signal.
 16. The method of claim 13 whereinthe time delay following receipt of the most recently received commandsignals is sufficient to allow a memory operation to complete.
 17. Themethod of claim 13 wherein the synchronous memory operation comprises aburst mode memory operation.
 18. An interface circuit for a mixed-modememory device, comprising: an asynchronous operation command detectioncircuit operable to generate a first memory operation activation signalin response to receipt of a command signal corresponding to a requestfor a memory operation; a synchronous operation command detectioncircuit operable to generate a second memory operation activation signalin response to receipt of a command signal corresponding to a requestfor a synchronous memory operation; a first logic circuit coupled to theasynchronous operation command detection circuit and the synchronousoperation command detection circuit, the first logic circuit operable toreceive the first memory operation activation signal and to generate anoutput signal after a time delay from receiving the first memoryoperation activation signal, the first logic circuit further operable toprevent the output signal from being generated in response to receivingthe second memory operation activation signal before the time delayends; and a second logic circuit coupled to the synchronous operationcommand detection circuit and the first logic circuit, the second logiccircuit operable to generate a memory access signal in response toreceiving the output signal and the memory access signal in response toreceiving the second memory operation activation signal.
 19. Theinterface circuit of claim 18 wherein the asynchronous operation commanddetection circuit comprises: a signal generator operable to generate asignal in response to receipt of the command signals corresponding to arequest for an asynchronous memory operation; and the first logiccircuit, the first logic circuit including a delay circuit coupled tothe signal generator and operable to generate the output signal a timedelay following receipt of a last received signal from the signalgenerator.